An experiment recently completed by the present investigators indicated that a cholinergic system was involved in the mediation of fear responses in the rat. Fear was defined as those behaviors (freezing, avoiding and suppressing consummatory responses) that are displayed by untreated rats in the presence of a cat. In Experiment 1, normal rats injected with either saline or the peripheral control (methylscopolamine) showed the above fear responses when confronted with a cat. In contrast, rats that had been injected with an anticholinergic drug, scopolamine, approached, sniffed and even drank in the presence of a cat. Animals injected with scopolamine spent significantly more time in proximity to the cat (even sniffing cat) than control animals (p is less than .01); spent significantly more time drinking in the presence of the cat (p is less than .05); and spent significantly less time freezing (p is less than .0001). Previous researchers have demonstrated effects of anticholinergic drugs on learning, memory and performance in a variety of shock avoidance paradigms. The results of Experiment 1 have shown that anticholinergic drugs also affect behavior in aversive situations that involved not conditioning of the aversive contingencies. Blockade of central cholinergic activity suppressed the animals' fear responses to a natural predator. The proposed research consists of a series of experiments that explore further the role of anticholinergic drugs in elaboration of fear responses. Experiments 2 & 3 will determine whether changes in emotional behavior are specific to a cholinergic system. Experiment 4 will explore a finding suggested by pilot work in which animals differing in speed of habituation to the open field test also differed in their responses to the drug injection. Experiment 5 will determine whether fear responses can be reduced by cholinergic blockade after animals have been exposed repeatedly to the cat. Experiment 6 will explore procedures designed to sustain drug-induced changes in fear responses on later nondrug tests.